Swend
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I'd say the opposite: If they don't have a common reference then there is nothing preventing them from having a common mode potential difference.
And if they're not galvancially isolated but you call them 'floating' that implies they do have a common mode potential difference.
Your circuit can't measure a signal that has greater than ~9V between your differential scope probe ground and the oscilliscope ground.
In my mind that's hardly a good differential probe and you havn't made a convincing case that this is actually ok in your application. Particularly given that you want to measure high voltages.
in relation to your scope ground (which is your differential probe circuit ground too).
Your 9V supplies are +/-9V in relation to "gnd".
"gnd" is tied to scope ground via J3 pin 2.
Your "Scope Probe" input J1 pin 2 is connected to U1 IN- via a 470ohm resistor.
U1 AD8130 as an absolute maximum rating of "−VS − 0.3 V to +VS + 0.3 V " on its input pins
So....that means J1 pin 2 needs to remain within +/-9.3V of "gnd" or scope ground.
It's not but J1 pin 2 is connected to U1's input. And that input can't be more than 9.3V from "gnd".
Yes that is correct. Now if I take J1 pin 2 and connect it to a the (-) of a 48V battery, and connect J1 pin 1 to the (+) of the same battery, what is the resulting voltage potential between U1 pin 1 and gnd? and the resulting voltage potential between U1 pin 8 and gnd? And can you please explain to me in technical terms why it is so?
If the circuit is galvancially isolated which it would be in the case of a battery powered circuit with no other references then you're good. The non-infinite impediance of your input circuit and the clamping diodes inside U1 will bring J1 pin 2 (battery -) to within +/-9.3V of 'gnd'.
But you'd also be just fine connecting (-) to your scope's ground in this case.
So we have an instance where your circuit is ok but isn't needed. Give an example where you think you need the differential probe functionality.
Because your differential scope probe inputs are clamped to within +/-9V of scope ground
It's like we are going round in circles now, either you want to discuss it and consider the possibility that you could be wrong - or if you just want me to agree with you without you presenting any valid arguments, we can also do that.
So for the last time on my part: In my circuit the scope probe inputs J1 and scope ground J3 are galvanically isolated by U1. That means I can connect my probes anywhere I want in a circuit as long as there is no common reference, including your modified example.
Can I do this with my circuit? Answer: Yes.
The AD8130 does not provide galvanic isolation!
If you want galvanic isolation you need an 'isolation amplifier' and an isolated supply to power it.
* you have the scope connected to EARTH GND.
EARTH GND and charger:
Even if the nodes are floating - or better: because of floating signals - there will be (a of voltage) between floting nodes and EARTH GND.
Not all customer´s will read and care about it.There will be a label on the enclosure of the diff-probe which reads "ONLY CHARGE WHEN PROBE AND SCOPE ARE DISCONNECTED!"
Your scope, yes. Others not.The scope (as in stand alone instrument) is NOT connected to EARTH (as in planet earth) and I'm using a isolation transformer.
Here is a simplified circuit modeling the path between the probe grounds of two of your circuits connected to a single scope.
V5 is like the battery in the example I gave above. It has no reference to scope ground (or earth) other than its connection to your circuit. This shows your circuit clamping voltage and drawing current (bad) when more than 18V is applied between two scope probe input grounds.
But there never is an ideal isolation. There always will be the possibility for current. Either resistive, or electrostatic or capacitive...
When you come close to EARTH GND there will be a spark and noticable current. Enough energy to destroy electronic devices.
Use the DVM and connect one wire to earth. AC voltage mode. Then do some tests and measure voltage at some electronic devices or other metals around that are considered to be "floating" or "isolated" with respect to EARTH GND.
What´s the voltage of your isolated scope?
Not all customer´s will read and care about it.
But even "isolated" ones will cause some current.
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